Bug fix for adding readgroup info

Previous awk statement (itself a replacement for novosort, which wasn't
obeying memory limits) wasn't applying readgroup properly to each record (and
STAR doesn't output HD SAM header).  FWIW, also considered Picard
AddOrReplaceReadgroup, but java has no way to limit CPU usage--so won't work
in our environment.  Also, Picard is actually slower than samtools sort,
despite using 10-15x more CPU.